1. Field of the Invention
The present invention relates to a backlight device and a display device and, more particularly, to a backlight device suitably used for a display device having a translucent display panel, and a display device having the backlight device.
2. Description of the Related Art
Some display devices including a non-self-emissive display panel such as a translucent liquid crystal display panel have a backlight device on the back side of the display panel. This backlight device includes a light source such as a fluorescent tube and predetermined kinds of optical sheets which control the properties of light emitted by the light source, and is arranged to illuminate the back side of the display panel with the light whose properties are controlled. The light illuminating the back side of the display panel passes through the display panel, making images displayed visible on the front side of the display panel, i.e., on the screen.
The optical sheets placed between the light source and the display panel is generally a stack of a diffusion sheet, a lens sheet, a prism sheet, and a polarizing reflection sheet.
Some optical sheets are stacks of a plurality of films or sheets having different coefficients of thermal expansion. When the display device is used and such optical sheets are heated by heat from the optical source and other members, a bend and/or a wrinkle may occur due to the difference in the coefficients of thermal expansion of the materials.
In addition, difference between the coefficient of thermal expansion of optical sheets and that of a member supporting the optical sheets may also cause a bend and/or a wrinkle of the optical sheets. In the case of a structure where the optical sheets are supported by the supporting member at the edge portion, when the optical sheets and the supporting member are expanded by heat, the vicinity of the region supported by the supporting member of the optical sheets is pulled due to the difference in the coefficients of thermal expansion, causing a bend and/or a wrinkle.
If a bend and/or a wrinkle occur in the optical sheets, the bend and/or the wrinkle may result in luminance nonuniformity and/or shades on the front side of the display panel. In particular, if the optical sheets bend repeatedly and thus undulates, or if a bend occurs in an optical sheet having directional optical properties such as a lens sheet, the luminance nonuniformity and/or shades on the screen are easy to visually perceive, which may degrade the display quality of the display device. Accordingly, it is preferable to keep the optical sheets free from a bend or a wrinkle.
Some conventional backlight devices are capable of reducing degradation of the display quality caused by a bend and/or a wrinkle of optical sheets. For example, a configuration is used in which a through hole is formed on the edge portion of optical sheets and a boss or other component is freely fitted into the through hole (see Japanese Patent Application Unexamined Publication No. Hei11-337942). In another configuration, a positioning piece is provided on the edge portion of optical sheets and is engaged into a concave portion provided in a chassis or a frame (see Japanese Patent Application Unexamined Publication No. 2001-76527). In yet another configuration, such a through hole or a positioning piece is used to support optical sheets at about the middle of the side which is located on the top in terms of direction of gravitational force when the display device is in use (see Japanese Patent Application Unexamined Publication No. 2005-158707). The above configurations allow the edge portion of the optical sheets to freely alter the shape by heat, thereby reducing occurrence of a bend due to thermal expansion of the optical sheets to some extent.
However, even with a backlight device having such a support configuration, there are some cases where a bend or a wrinkle cannot be prevented perfectly, depending on the conditions of thermal expansion of the optical sheets.
For example, if the temperature of the edge portion of the optical sheets differs from that of the center portion, the amount of thermal expansion may be nonuniform in accordance with the temperature distribution, which may result in nonuniform bends and/or wrinkles. If the bending states are different as described above between the edge portion and the center portion of the optical sheets, it is difficult to remove the bends and/or wrinkles even if the edge portion of the optical sheets can freely alter its shape.
As described above, various factors such as the coefficient of thermal expansion and temperature distribution affect the optical sheets in combination and cause various types of bends. Consequently, it is very difficult to entirely remove bends and wrinkles of the optical sheets.